As the degree of integration of semiconductor devices on a substrate increases and the linewidth of conductive lines (e.g., gate electrodes) decreases to about 0.25 .mu.m and below, the use of conductive lines formed of doped polycrystalline silicon (polysilcon) may become problematic. In particular, as the linewidth of polysilicon conductive lines is reduced, the resistance of polysilicon conductive lines is increased and may become sufficiently high so as to significantly increase the transmission delay associated with signals applied thereto even if the polysilicon lines are highly doped.
To overcome such problems, attempts have been made to form polysilicon conductive lines having low resistivity and a work function potential which corresponds to the midgap work function potential of silicon. Recently, conductive lines consisting of a stacked structure of a polysilicon layer and a thermally treated silicide layer have been proposed. For example, conductive lines consisting of tungsten silicide (WSix) or titanium silicide (TiSix) have been widely used. Metal silicides having high melting temperatures, such as cobalt silicide CoSix, have also been used. Among them, TiSix is typically considered superior in terms of thermal stability and barrier characteristics and also typically has a resistivity of about one fourth (1/4) the resistivity of WSix. Accordingly, TiSix has typically been considered as a potential gate electrode material for 1-gigabit DRAM devices and even more highly integrated devices. Moreover, in the event TiSix is used as a gate electrode of a field effect transistor, it may be possible to obtain excellent transistor characteristics since TiSix typically has a low resistivity and a work function potential which corresponds to about the midgap work function potential of silicon.
Referring now to FIGS. 1A-1C, cross-sectional views of intermediate structures illustrate a method of forming metal silicide conductive lines according to the prior art. Referring specifically to FIG. 1A, a conductive polysilicon film 6 is formed after forming an oxide film 4 on a face of semiconductor substrate 2. A layer 8 of TiSix is then formed by reacting titanium with the polysilicon film 6 using a thermal treatment step or by sputter depositing a layer of TiSix directly, for example. An insulating film 10 (e.g., SiO.sub.2, Si.sub.3 N.sub.4) is then deposited on the layer 8 of TiSix. Referring now to FIG. 1B, a photoresist pattern 12 is then formed on the insulating film 10 using a photolithography process. Referring now to FIG. 1C, a TiSix film pattern 8a and a polysilicon film pattern 6a are formed by etching the insulating film 10 and then etching the TiSix film 8 and the polysilicon film 6 using the photoresist pattern 12 as an etching mask.
The step of etching the TiSix film 8 and the polysilicon film 6 may be accomplished by performing a dry etching step using a fluorine-containing gas such as SF.sub.6 or CF.sub.4, a chlorine-containing gas such as HCl, Cl.sub.2 or BCl.sub.3, or a gas such as Hbr. Unfortunately, the fluorine-containing gases and the chlorine-containing gases are typically not suitable as etching gases because they also cause etching in a lateral direction at the interface between the TiSix film 8 and the polysilicon film 6. As will be understood by those skilled in the art, conductive materials may also remain at the etched interface and these conductive materials may result in the formation of stringers and bridge defects which can reduce device yield.
In the event HBr is used as an etching gas, it may be hard to control a critical dimension (CD) of the TiSix film pattern 8a and the polysilicon film pattern 6a since a substantial amount of a polymer by-product (which is a non-volatile residue) may be generated by the reaction between the TiSix film 8 and the etching gas. In the event Cl.sub.2 is used solely as an etching gas, the lateral etching phenomenon may not occur when the photoresist pattern 12 is used as a mask, however, lateral etching may occur when a hard mask, other than a photoresist pattern, is used.
Thus, notwithstanding the above described methods, there continues to be a need for improved methods of forming electrically conductive lines containing silicide layers.